Cigarette smoking early in pregnancy slightly disturbs maternal thyroid hormone levels that are important for a healthy baby, according to a large study being presented Sunday, June 3, at The Endocrine Society’s 89th Annual Meeting in Toronto.

This effect is not seen in women who stop smoking during pregnancy or who never smoked, said author Dr. Bijay Vaidya of Peninsula Medical School in the United Kingdom.

Past research has shown that smoking mildly stimulates production of the thyroid hormones in the general population. Until now, however, smoking’s effect on thyroid hormones during pregnancy was not known, said Vaidya, a consultant endocrinologist and honorary clinical research fellow.

During pregnancy, substantial excess of maternal thyroid hormone is associated with a higher risk of miscarriage, premature birth and low birth weight, he said.

The two thyroid hormones produced by the thyroid gland, triiodothyronine and thyroxine, regulate metabolism—how the body turns food into energy. When these hormones are higher than normal and the hormone that stimulates the thyroid to produce them (thyroid-stimulating hormone, or TSH) is too low, the thyroid gland is considered overactive. This condition is called hyperthyroidism. Symptoms of hyperthyroidism include a rapid heartbeat, fatigue, weight loss, increased sweating and irritability.

Vaidya’s study included 1,428 women who had no obvious thyroid disorders and whose pregnancy ranged from six to 22 weeks’ gestation. More than 54 percent were nonsmokers, 28 percent were current cigarette smokers and 17 percent were smokers who stopped smoking during pregnancy. The level of triiodothyronine was much higher and TSH was much lower in current smokers than in the other groups of women. This difference did not depend on the number of cigarettes smoked, the researchers found. However, smokers’ higher levels of thyroid hormones, while statistically significant, were not great enough to be considered thyroid disease.

“It is unknown whether subtle excess in thyroid hormone levels due to smoking, as seen in our study, causes adverse pregnancy outcomes,” Vaidya said. “It is possible that some of the known effects of smoking during pregnancy, for example, low birth weight, may occur through smoking’s stimulatory effect on the thyroid hormones.”

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